Until very recently, the primary flight control systems of commercial aircraft have utilized mechanical cables to transmit pilot-produced control inputs to the control surfaces of the aircraft. Pilot-produced inputs are created by a pilot moving various pitch, roll and yaw axis control devices, such as the column, wheel and rudder pedals located in front of the pilot (and copilot) seat(s) in an aircraft cockpit. The control surfaces of the aircraft include the elevators, ailerons, spoilers and rudder of the aircraft. In operation, a pilot manually "flies" an aircraft by moving various pitch, roll and yaw axis control devices so as to position the control surfaces in a way that makes the aircraft follow a desired flight path through space. Aircraft weight, center of gravity location, and aerodynamic configuration and location in a flight envelope determine how the pilot positions the pitch, roll and yaw axis control devices in order to follow a desired flight path. Changes in any of these factors may require that the position of the control devices be different, even when performing the same maneuver. Especially in adverse weather conditions (e.g., turbulence, wind shear, precipitation and poor visibility), pilot workload from manually operating pitch, roll and yaw axis control devices, plus navigation and other equipment, can become excessively high. Excessive pilot workload has the possibility of compromising safety, particularly during critical portions of a flight, such as approach, flare and landing.
Over the years, flight control system improvements, such as hydraulically powered control surfaces, "feel" systems, ratio changers and yaw dampers have helped to reduce pilot workload and to provide a more uniform aircraft response to given pilot control inputs when an aircraft is operating with different center of gravity locations and in different parts of its flight envelope. While these improvements have helped to reduce pilot workload under normal operating conditions, they have only partially solved the pilot workload problem in adverse weather conditions. The pilot workload problem in adverse weather conditions has been only partially solved because, even with these improvements, the basic way a pilot flies an airplane has remained unchanged--the pilot still commands control surface positions directly by positioning pitch, roll and yaw axis control devices.
The generation of commercial transport aircraft presently being developed will feature electronic flight control systems that are expected to provide a quantum reduction in pilot workload and a quantum improvement in flying qualities. Electronic flight control systems, such as fly-by-wire (FBW) and fly-by-light (FBL) flight control systems, will permit a pilot to command parameters other than control surface position with available control devices. For example, one system presently being considered for use on the next generation of Boeing aircraft allows a pilot to command pitch attitude rate-of-change through a pitch-axis control device. The pitch-axis control device is presently contemplated to be in the form of either a conventional control wheel column or a sidestick controller. Based on the pilot's positioning of the control device, the electronic flight control system will command the elevators of the aircraft to move in the manner required for the aircraft's actual pitch attitude rate-of-change to follow the pilot's commanded pitch attitude rate-of-change. In other words, a given input on the pitch axis control device will command a given rate-of-change of the pitch attitude of the aircraft. Returning the control device to a neutral position will zero the rate-of-change, not the pitch attitude. As a result, when the control device is placed in a neutral position, the aircraft will maintain the previously set pitch attitude. Changing from one pitch attitude to a different pitch attitude will require that a pilot move the control device to cause the pitch attitude to decrease (or increase) until the desired pitch attitude is achieved. That is, movement of the pitch axis control device will cause a pitch attitude change to occur that will ultimately result in the aircraft reaching level attitude (if this is the desired pitch attitude), at which time the control device will be moved to its neutral position. The magnitude of pitch axis control device movement will control the magnitude of the rate-of-change of pitch attitude.
The foregoing system reduces pilot workload because the electronic flight control system forces the aircraft to follow the pilot's pitch attitude rate-of-change commands regardless of aircraft inertia or aerodynamic configuration, location in the flight envelope, or the presence of external disturbances such as turbulence and wind shear. While a pitch attitude rate-of-change electronic flight control system is expected to considerably improve pitch-axis flying qualities during the takeoff, climb, cruise, and descent portions of a flight, the use of pitch attitude rate-of-change commands during the flare portion of a landing poses problems in two specific areas. First, in a conventional flare maneuver, a pilot pulls back on a control column and holds it back until the main wheels of the aircraft touch the runway, i.e., the aircraft lands. This action causes a pitch attitude change that is dependent on how far the stick is pulled back and held. In a pitch attitude rate-of-change electronic flight control system, a pitch attitude change is caused by applying a control pulse, not a steady pull and hold, to a pitch axis control device. This procedural difference requires pilot retraining and the additional expense associated with such retraining in order for a pitch attitude rate-of-change electronic flight control system of the type described above to be used during the flare portion of a landing. The second problem area relates to the risk of overflaring. The risk of overflaring is increased substantially using a pitch attitude rate-of-change electronic flight control system if a pilot does not return the pitch axis control device to its detent (i.e., neutral) position at the precise time the pitch attitude of the aircraft achieves the angle that produces a low sink-rate landing. If the pitch attitude changes, a pilot using a pitch attitude rate-of-change electronic flight control system is required to move the pitch axis control device to a position that creates the pitch attitude required to continue a low sink-rate landing. In summary, unconventional pilot maneuvering techniques during the flare portion of a landing are required with a pitch attitude rate-of-change electronic flight control system of the type described above.
One obvious way to avoid the foregoing problem is to deactivate the pitch attitude rate-of-change electronic flight control system during the flare portion of a landing. This approach has the disadvantage of losing all of the turbulence rejection, configuration masking and flight-envelope location masking effects normally provided by an electronic flight control system. Because flare is a high pilot workload portion of a flight, the loss of handling qualities and turbulence rejection benefits during landing flare is highly undesirable.
One proposed approach to avoiding the foregoing and other problems is to provide a flare control modification for pilot-in-the-loop aircraft maneuver command electronic flight control systems. During flare, pilot pitch control commands are interpreted as incremental flight path angle commands above a reference flight path angle, nominally a -3.degree. glide slope. The flare control modification allows conventional piloting techniques (i.e., pitch controller pull-and-hold) to be used during flare while retaining the benefits of the maneuver command system masking effects of gusts, winds, wind shear, and variations in airplane weight, balance, and aerodynamic configuration. Such an approach is described in U.S. application Ser. No. 282,265, entitled "Flight Path Angle Command Flight Control System for Landing Flare" by Messrs. Sankrithi and Pelton, filed Dec. 8, 1988, and assigned to the assignee of the present invention. While such an approach solves the foregoing problems, it has two perceived disadvantages. Specifically, many pilots feel that their task changes from one of controlling flight path directly to one of controlling flight path indirectly by direct control of airplane attitude during landing flare. This "feeling" results from the fact that, as the ground approaches, pitch attitude cues become very intense as a pilot looks out the windows of an aircraft. Experienced pilots have learned to accurately land an aircraft using attitude cues and attitude controls. Thus, many pilots feel that a direct pitch attitude control system is more appropriate than a flight path angle control system during landing flare. A second perceived disadvantage of a flight path angle control system is associated with the fact that pilots use thrust control in combination with pitch control to obtain desired landing performance. In many scenarios using prior art (e.g., non-electronic) control systems, a pilot establishes a proper landing attitude with a given stick deflection followed by a reduction in throttle setting. This results in the aircraft "sinking" to touchdown. Sinking is accomplished at a nearly constant pitch attitude (and a nearly constant stick deflection). The dynamics of the situation involve an angle of attack increase and a corresponding flight path angle decrease (i.e., flight path angle becomes more negative) as speed bleeds off and pitch attitude is held nearly fixed. With an incremental flight path angle control system, as speed bleeds off with a fixed stick deflection, the flight path is maintained, and the airplane only slows down. Both attitude and angle of attack increase slightly. To increase sink rate, the pilot must relax stick force. The overall net effect of direct flight path angle control during flare is a tendency to "float". This results in a large percentage of touchdowns occurring beyond the desired touchdown spot.
The present invention is directed to avoiding the foregoing and other problems by providing a flare control modification for a maneuver command electronic flight control system that maintains the advantages of the system while allowing the pilot to use conventional piloting techniques during the flare portion of a landing. More specifically, the present invention is directed to providing a pitch axis command electronic flight control system for landing flare that reduces the risk of overflaring during the flare portion of a landing while retaining the turbulence rejection, configuration masking and location in the flight envelope masking benefits produced by closed-loop electronic flight control systems.